1. Field of the Invention
The present invention relates to a technology for fixing a toner image on a recording medium in an image forming apparatus.
2. Description of the Related Art
In recent years, there is an increasing demand for a high-speed process of an image forming apparatus, such as a printer, a copier, a facsimile machine, that forms an image using an electrophotographic method. For example, in an image forming apparatus including a heat-roller-type fixing device that applies heat and pressure onto a recording medium such as a sheet of paper or film on which a toner image is formed, a plurality of heating units is provided in the fixing device or a large amount of power is supplied to each heating unit, thereby meeting the demand for high-speed image formation.
An increase of the speed of an image forming apparatus is increased by supplying a larger amount of power or by providing a plurality of heating units increases the required power, resulting in a possible voltage fluctuation (hereinafter, “flicker”) of the image forming apparatus or the fixing device. Especially when the image forming apparatus or the fixing device is switched from the OFF-state to the ON-state, an inrush current flows, which is several times higher than a current in a steady state, and the flicker gets worse. In an image forming apparatus, generally, in an operating mode in which an operation, such as copying and printing, is performed and in a standby mode in which the apparatus is ready for taking an operation, the flicker is controlled. However, in a place such as a typical office, an image forming apparatus remains for a longer time in the standby mode than in the operating mode. Therefore, the flicker needs to be controlled more strictly in the standby mode. The flicker may cause influence on power supply to peripheral devices to which power is supplied from a common power source.
Japanese Patent Application Laid-open No. 2003-217793 discloses a heater control device that, when a plurality of heating units is switched on, applies current to the heating units not simultaneously but with a time lag to independently soft start and switch on the heating units, and that, when the heating units are switched off, soft stops and switches off the heating units with a time lag.
In the heater control device disclosed in Japanese Patent Application Laid-open No. 2003-217793, however, even if the timing at which power supply to each of the heating units starts is shifted sequentially, the heating units tend to repeat a cycle of switching on and off each heating unit in the same cycle. Moreover, because the heating units are thermally stable in the standby mode in which the heat is not transferred to a recording medium from the fixing device, a period during which the heating units are switched off gets longer. Therefore, at the time of switching on the heating units again, the inrush current may be higher than the current in the operating mode. As a result, it is difficult to suppress the flicker. In an image forming apparatus in which the amount of power to be supplied per each heating unit is large, the flicker gets even worse. In the standby mode in which the heat is not transferred to a recording medium from the fixing device, the flicker is required to be suppressed more strictly, as described above. Therefore, the flicker caused by the heating units repeating the cycle of switching on and off is required to be suppressed more efficiently.